1. Field of the Invention
The present invention relates to a cylindrical part manufactured by fiber reinforced plastic composite material so as to be used as a structural member, and a method of manufacturing the same cylindrical part by fiber reinforced plastic composite material.
2. Description of the Related Art
The fiber reinforced plastic composite material is excellent in specific strength, specific rigidity and corrosion resistance. However, there still exist various technical problems to be solved when a structural member is manufactured by fiber reinforced plastic composite material.
When a thick-walled cylindrical part is manufactured by the fiber reinforced plastic composite material, various methods such as the filament winding method, tape winding method, sheet winding method, and etc. are now being adopted.
However, the fiber reinforced plastic composite material is provided with such characteristics that a thermal expansion coefficient thereof differs largely according to the lamination direction. Therefore, when a thick-walled cylindrical part is manufactured by the fiber reinforced plastic composite material, internal stress proportional to a difference between the curing temperature and room temperature is inevitably generated in the fiber reinforced plastic composite material, after being heated for curing. In the case of the thick-walled cylindrical part, there exists the case where the internal stress generated in the fiber reinforced plastic composite material exceeds an interlayer bonding strength of the fiber reinforced plastic composite material. In this case, an interlayer separation occurs in the fiber reinforced plastic composite material.
In a general laminating process of thick-walled cylindrical parts, many lamination angles of .+-.10.degree./.+-.45.degree./.+-.85.degree. are combined with each other. In this case, when a diameter of the cylindrical part is small (e.g., less than 200 mm) and the wall thickness is large (e.g., more than 30 mm), since the internal stress becomes excessively large, there exists a problem in that the interlayer separation occurs.
To overcome this problem related to the interlayer a separation, Japanese Published Unexamined (Kokai) Patent Application No. 2-236014 discloses such a method of laminating a layer strengthened against torsion (of which the lamination angle is .+-.30.degree. to .+-.60.degree. with respect to the axial direction) and the layer strengthened against bending (of which the lamination angle is 0.degree. to 20.degree.) alternately, in order to suppress the interlayer separation due to a difference in the thermal expansion coefficient between the conditions when heated for curing and when cooled to the room temperature.
Further, Japanese Published Unexamined (Kokai) Patent Application No. 6-335973 discloses a method of using high frequency induction heating means, to reduce the temperature dispersion while heating for curing and thereby to decrease the internal stress thereof.
In the first prior art method of laminating plies at different lamination angles (i.e. directions), since the wall thickness of the manufacturable cylindrical parts is limited to about 15 mm, when the wall thickness thereof increases more than this value (e.g., 15 to 50 mm), the internal stress generated in the fiber reinforced plastic composite material sharply increases with increasing wall thickness, so that the interlayer separation occurs in the fiber reinforced plastic composite material layers. Therefore, it is impossible to manufacture a cylindrical structure of high strength and high rigidity by making the best use of the characteristics of the composite material.
Further, in the second prior art method of using a high frequency induction heating apparatus in order to reduce the temperature dispersion during the heat curing and thereby to reduce the internal stress generated in the fiber reinforced plastic composite material layers, since magnetic substances are added to the matrix resin, the weight thereof inevitably increases, and as the result there exists a problem in that the weight of the structural body is increased and further the performance of the structural body is degraded. In this method, additionally there exists another problem in that a special high frequency induction heating facility must be prepared in accordance with the shape of the manufactured structural body, instead of an autoclave or a heating furnace.
Further, when a mandrel formed of steel (thermal expansion coefficient: 10 to 12.times.10.sup.-6 /.degree.C. ) is used, since the mandrel is shrunk when cooled after heat curing, there exists a problem in that the inside layers are shrunk in the radial direction thereof and thereby the interlayer separation often occurs.